This invention relates to an electric motor driven pump and more particularly to a pump for pumping hazardous liquids, such as petroleum and chemical products, from the cargo area of a tanker ship, for example.
The transportation of bulk quantities of various petroleum products, including crude and refined derivative fluids, liquefied gases and a wide array of chemicals has been accomplished by ocean transport for decades. Many of these fluids are hazardous to humans and the environment by being toxic, explosive or both. Once the cargo is delivered to a receiving terminal, it is necessary to pump the cargo from the tanks and then through piping to land based storage facilities. Because of the hazardous characteristics of the cargos, specialized centrifugal pumps have been designed and developed to safely accomplish this function.
One method of pumping hazardous liquids from ocean tankers has been to use electric motor driven deep well pumps. These pumps include an electric motor of explosion proof design, which is mounted on the tanker deck, that is connected to a centrifugal pump located in a sump at the bottom of the cargo tank. It will be appreciated that the electric motor must be separated from the hazardous liquid to be pumped in order to prevent explosion which may be caused when a spark is produced by the failure of the electric motor. Thus, the motor and the pump are connected by a long shaft. Numerous sleeve-type bearings are required to position and support the long shaft. It is also difficult to maintain the necessary alignment of these bearings and thus maintenance must be frequently performed on these components. The sleeve-type bearings are lubricated by the fluid being pumped. Thus, the useful life of these bearings is influenced by the lubrication characteristics of the liquid and/or the amount and characteristics of foreign particles carried by the liquid.
The only currently accepted use of an electric motor prime mover close coupled to a centrifugal pump which is submerged in the pumped hazardous liquid is for liquefied gas applications. In this specific application, the motor cavity of the device is flooded with the pumped hazardous liquid. The absence of an oxidizer in the presence of a potential electrical discharge eliminates the hazard of an explosion. However, a significant weakness of this design is that the lubricating properties of the pumped hazardous liquid are quite poor for the rolling element bearings which are used for this application. Bearing failures are not uncommon. Furthermore, for safety reasons, operation of the pump is limited by the requirement that the motor must be de-energized once the cover gas above the hazardous liquid free surface starts to be drawn into the pump. It is standard practice not to empty the tank. In this manner, the operational temperature and the oxidizer deleted vapor within the tank are maintained.
Close coupled, hydraulic motor driven centrifugal pumps are permitted to be operated while submerged in the cargo tank. There are no sources of ignition when using this pumping system. However, a high pressure (up to 3,000 psi), complicated and difficult to maintain oil system is required to provide the energy to the hydraulic motor.
Thus, there remains a need for a electric motor driven pump that can be safely used to pump hazardous liquids from a product or chemical tanker.